A series of studies in the rat have been utilized to ascertain the extent to which testosterone and estradiol can regulate the secretion of FSH and LH in the absence of the testes. The data demonstrate the 1) testosterone alone at physiologic doses can maintain both gonadotropins within the normal range, 2) that estradiol alone preferentially suppresses LH, but not FSH concentrations and 3) that a selective increase in plasma FSH is produced when there is reduced androgen production in association with increased estradiol production. Similar changes are seen in the pituitary content of FSH and LH suggesting that these altered sex steroid levels effect gonadotropin synthesis, strage and release. These data provide an alternate explanation for the inhibin hypothesis which suggests that there is a specific non sex steroid inhibitor of FSH secretion arising from the seminiferous tubules.